The present invention relates to a continuous process for preparing lower dialkyl carbonates as main product and alkylene glycol as by-product by catalyzed transesterification of a cyclic alkylene carbonate (e.g. ethylene carbonate or propylene carbonate) with lower alcohols, where the reaction of the alkylene carbonate is carried out with an alcohol containing dialkyl carbonate in countercurrent, characterized in that introduction of a stream containing at least 99.5% by weight of alcohol takes place below the point of introduction for the alcohol containing dialkyl carbonate in a particular spacing ratio between the abovementioned points of introduction.
The preparation of dialkyl carbonates from cyclic alkylene carbonate and alcohol, in which alkylene glycol is simultaneously formed as by-product, is known and has been widely described. In U.S. Pat. No. 6,930,195 B2, this catalyzed transesterification reaction has been described as a two-stage equilibrium reaction. In the first reaction stage, the cyclic alkylene carbonate reacts with alcohol to form hydroxyalkyl carbonate as intermediate. The intermediate is then converted by means of alcohol in the second reaction stage into the products: dialkyl carbonate and alkylene glycol. Both the quality of the by-product (alkylene glycol) and the quality of the dialkyl carbonate play a very important and decisive role for the development of an economically attractive process for preparing dialkyl carbonates. There is therefore an urgent need for a production process which can produce the alkylene glycol with a very low impurity content.
For the industrial implementation of the dialkyl carbonate production process, the use of a reactive distillation column, which has been described, inter alia, in EP 569 812 A and EP 1 086 940 A, has been found to be particularly advantageous. In EP 569 812 A, the cyclic alkylene carbonate is fed continuously into the upper part of the column and the alcohol containing dialkyl carbonate is fed continuously into the middle or lower part of the column. In addition, pure alcohol is introduced below the point of introduction of the alcohol containing dialkyl carbonate. The low boiler mixture, which comprises the dialkyl carbonate produced, is taken off continuously at the top of the column and the high boiler mixture, which comprises the alkylene glycol produced, is taken off continuously at the bottom of the column.
In EP 1 086 940, the preparation of the dialkyl carbonate was also demonstrated using a reactive distillation column. Here, the arrangement of the point of introduction of the starting material and the offtake point for the product along the reactive distillation column is similar to that in EP 569 812 except that here the additional introduction of pure alcohol in the lower region of the column has been omitted.
It has been established that the difficulties in respect of maintaining the high quality requirements for the by-product alkylene glycol can be solved particularly simply and advantageously when the content of unreacted alkylene carbonate is very small not only in the subsequent work-up steps but also immediately after the transesterification reaction.
It has been found that the impurities can be reduced only with an increased outlay (e.g. in terms of energy) by the use of the arrangement of the points of introduction of the starting materials described in EP 1 086 940. The additional use of a second point of introduction of alcohol below the point of introduction for the alcohol containing dialkyl carbonate does not on its own make maintenance of the desired purity of the products possible. In addition, this disclosed process requires the use of pure alcohol in the second point of introduction of alcohol.
There was therefore a need for a continuous process for preparing lower dialkyl carbonates as main product and alkylene glycol as by-product by catalyzed transesterification of a cyclic alkylene carbonate with lower alcohols, in which the amounts of cyclic alkylene carbonate are very low.